The present invention is directed to a hand-held testing device for indicating different values of an electric test voltage.
Hand-held testing devices are well known in the art and frequently used to display the status of voltages with different applications. One of these testing devices is known from the German Pat. No. 20 60 884. This known testing device is adapted for indicating either the polarity or the phase of an electric voltage relative to a given reference voltage. It has a grip body with a connecting point to receive the test voltage. This connecting point is connected by means of a current limiting resistor to a network of two light emitting diodes arranged in anti-parallel. The antiparallel connection of light emitting diodes is, in turn, connected to a cable forming, for example, a connection to ground or another connecting point for receiving the test voltage. When a direct current (d-c) voltage is tested just one of the light emitting diodes is switched into operable condition and is illuminated. When one of the light emitting diodes is illuminated depends upon the polarity of the test voltage which is received at the connecting point. If an alternating current (a-c) voltage is tested, both light emitting diodes are illuminated.
The current limiting resistor of the electric circuit may be designed as a ceramic PTC resistor. This known type of resistor consists of ferroelectric material on a substrate of n.sup.- and/or p.sup.- doped barium titanate and has, in the range of the Curie temperature, a sudden increase of its electric resistance by a factor or 30 or more. If such a ceramic resistor is used with the testing device, test voltages of about 2 V up to 500 V can be analyzed. In the known testing device, the ceramic resistor performs a protective function for the display elements, as it limits the current by self-heating beyond the Curie temperature.
Another testing device is known from the German Auslegeschrift No. 27 17 826. This known testing device has two grip bodies connected by a cable, each grip body comprises a testing tip and parts of an electric circuit. This testing device is adapted for indicating an electric voltage and/or its polarity or the condition of conductivity of an electric conductor. One of the grip bodies contains a battery and a protective diode connected therewith in series. The ends of this series connection are connected to respective connectors of a double throw switch which is designed as a push-button switch. As one of the input connecting points is also directly connected to one of the connectors of the double throw switch, this switch bridges the battery and the protective diode in one position, which is defined to be the inoperative position. This circuit arrangement of a push-button switch, a battery and a protective diode is in turn, connected via an antiparallel connection of two light emitting diodes and a current limiting resistor to the second input connecting point.
This construction of the known testing device is useful for a variety of applications in practice. The testing device may analyze the presence of an electric voltage and its polarity and may analyze the condition of an electric conductor for passing a current. However, this testing device has no feature to obtain a measurement of the value of the received test voltage.
On the other hand, there is a need to obtain, when testing for the presence of an electric voltage, also approximate information about the value of the test voltage. At high voltages often it is reasonable to separate, whether the value of the test voltage is 100 V, 220 V or 380 V. Also, there are many applications within a lower range of voltage where it is useful to know whether the voltage is, for example, 3 V, 6 V, 12 V or 24 V. While there are a variety of devices for these devices for these purposes with which the exact value of the voltage can be measured, the circuitry of such instruments is expensive and, therefore, the prices of these measuring devices are high. However, in many cases of voltage testing, it is not necessary to obtain an exact value of the voltage, because in a majority of testing situations it has just to be decided whether a minimum voltage is exceeded.
Additionally, from the German Offenlegungsschrift No. 25 03 855 it is known to provide an electric circuit arrangement forming a display section by means of a series connection of light emitting diodes. However, this known circuit arrangement has not been used in reference to a hand-held testing device as described hereinbefore. This circuit arrangement is known in reference to monitoring or supervising desks of TV or record studio facilities. In this circuit arrangement, each of the light emitting diodes are connected in series to a circuit which is supplied by a constant current source. Each of the light emitting diodes is connected in parallel to the emitter-collector-path of respective transistors which control the operable condition of the associated light emitting diode. For the aforementioned purpose series connections of a multiplicity of light emitting diodes are often used where the number of light emitting diodes in operable condition increases linearly with the low value of the voltage to be indicated. Therefore, the bases of the switching transistors are connected to the outputs of respective operational amplifiers. Each operational amplifier is connected by its non-inverting input to a connector tap of a voltage divider. Inverting inputs of the operational amplifiers commonly receive the test voltage to be displayed.
This circuit arrangement gives a satisfactory control of the light emitting diodes. This is essential as these display elements are very sensitive to excessive voltages. This is obtained from the control of the current which passes through all light emitting diodes in the operable condition, which current is limited to the minimum value necessary for the operation of the diode. The circuit arrangement is such that this feature is to be obtained, regardless of the sequence and number in which the light emitting diodes are switched on. The current control circuit consists of another transistor which is connected in series by its collector-emitter-path to the parallel series connections of the light emitting diodes and the amplifying transistors. The emitter is connected to an emitter resistor, while the base of the transistor is connected to an adjustable tap of a voltage divider, which is served by a supply voltage. The circuit of display stages with the series connections of the light emitting diodes and the amplifying transistors further includes a zener diode, which is to ensure reliable switching of the amplifying transistors.
This well-known circuit arrangement has been used up to now, as previously mentioned, only in reference to monitoring desks. This specific application has specific requirements. Above all there are used long chains of display stages which are switched one after another into an operable condition with increasing test voltage. This specific use of the known circuit arrangement in measuring instruments of high reliability has no relation to a hand-held testing device where the application requirements are totally different.